1. Field of the Invention
The present invention relates to a semiconductor device having a field effect transistor in which a current flows in a specified direction.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional semiconductor device having a field effect transistor to which a diode is connected, and in which a current flows in a specified direction. This semiconductor device comprises semiconductor substrate 1, N.sup.- type epitaxial layer 2 formed on substrate 1, and P type element-isolating layer 7 formed on substrate 1 and separating N.sup.- type epitaxial layer 2 into two portions. A field effect transistor is formed in the first portion of N.sup.- type epitaxial layer 2. The field effect transistor comprises P-channel region 3 formed in the first portion of epitaxial layer 2, N.sup.+ type source region 4 formed in P-channel region 3, N.sup.+ type drain region 5 in the first portion of epitaxial layer 2, insulation film 11 formed on epitaxial layer 2 and element-isolating layer 7, and gate electrode 6 formed on insulation layer 11 and located above N.sup.+ type source region 4.
A diode is formed in the second portion of N.sup.- type epitaxial layer 2. This diode comprises N.sup.+ type cathode region 9 and P.sup.+ type anode region 8. N.sup.+ type cathode region 9 is electrically connected by aluminum wire 10 with N.sup.+ type drain region 5 of the field effect transistor.
When a current flows in the forward direction within the semiconductor device shown in FIG. 1, it flows into the drain region 5 of the field effect transistor through P.sup.+ type anode region 9, N.sup.+ type cathode region 8, and aluminum wire 10. Since the field effect transistor and the diode are each formed two epitaxial layers both formed on the same substrate, another current may flow, in some cases, from P.sup.+ type anode region 8 into N.sup.+ type drain region 5 of the field effect transistor through N.sup.- type epitaxial layer 2 and P type element-isolation layer 7, due to a surge voltage generated when the semiconductor device is switched on or off or when the power-supply voltage of the device varies. In other words, a current may flow through epitaxial layer 5 and isolation layer 7 as if the PNPN thyristor parasitic in this device were turned on. This phenomenon is generally known as "latch up."
Generally it is difficult to prevent a latch-up from taking place within semiconductor devices. When a latch-up occurs in a semiconductor device, heat is generated in the device, destroying the elements formed within the device in some cases.